From recent congressional testimony (pdf):
A variety of demand, supply, and institutional forces have been at work in reducing young employment opportunities. Unprecedented levels of legal, illegal and temporary immigration have been one of the factors underlying this deterioration in youth labor markets. Declines in youth employment have been matched almost one for one with increased employment of new arrivals over the past 7 years. This summer we project that U.S. labor markets will have the lowest rate of teen employment since we have kept data going back to 1948.
Jobs Americans won't do? Hah!
Go to Iowa, Maine, North Dakota, Idaho, and Nebraska where there are relatively few guest workers or immigrants, and you will find much higher rates of teen employment (in the mid to high 50s), as well as teens working in the types of jobs that it is claimed no one wishes to take.
Here's an earlier piece on the same topic.